1. Field of the Invention
The present invention relates to a digital exchange system, more particularly, to a method for dealing with two channel signals having different bit rates.
Information is rapidly becoming a marketable commodity around the world. Japan is now testing a highly advanced "information network service" (INS) system. Other countries are trying out the widely known "integrated service data network" (ISDN) on a commercial basis. The digital exchange system according to the present invention is applicable to both INS and ISDN. The following explanation primarily refers to ISDN.
2. Description of the Related Art
In an ISDN, special transmission lines are set aside to connect subscribers and a plurality of digital exchange offices. Each transmission line includes two identical "B" channels and one "D" channel, i.e., signals pass through 2B+D channels. According to CCITT protocol, the B channels transmit channel signals at a bit rate of 64 kbit/s. Usually, the B channels are used for transmission of 8 bit voice information signal obtained with 8 kHz sampling.
In the same protocol, the D channel transmits channel signals at a bit rate of 16 kbit/s. Usually, the D channel is used for transmission of control signals.
B and D channel signals from subscribers are transferred to a digital exchange office where the required switching operations are performed. The switching operations are carried out at a bit rate of 64 kbit/s in view of the maximum 64 kbit/s bit rate of the B channel signals. Accordingly, no adjustment in bit rate is needed at the digital exchange office, for the B channel signals. Adjustment for the D channel signals, however, is inevitable, due to the low 16 kbit/s bit rate.
To match the bit rates of the B channel signals and D channel signals, the inventors have previously attempted to reform the bit stream of the D channel signals to adjust the bit rate of the D channel signals to substantially that of the B channel signals. For example, they have reformed a digital D channel signal having a logic sequence of "10" at 16 kbit/s to one having a logic sequence of "1111 0000" at 64 kbit/s. In other words, they changed the original D channel signal "10" to match the B channel signal without changing the inherent logic information ("10") by increasing the original bit rate for the D channel signal four ##EQU1## times.
This is, however, clearly disadvantageous in terms of the transmission efficiency of the digital exchange system because the reformed D channel signal, e.g., "11110000" comprises only two useful bits "1" and "0", and three useless "1" bits and three useless "0" bits.